1. Field of the Invention
The present invention relates to a zoom lens system, and particularly to a zoom lens system preferably usable in image pick-up apparatuses, such as a digital still camera, and a video camera.
2. Related Background Art
In recent cameras using solid-state image pick-up devices of CCD sensors, CMOS sensors, or the like, such as a video camera, a digital camera, and an electronic still camera, performance has been enhanced. In conformity therewith, high optical performance and reduced size are both required for optical systems used therein.
Generally, as a zoom lens used in image pick-up apparatuses using solid-state image pick-up devices, such as a video camera, often used is a lens type including three to five lens units, in which a lens unit disposed on the side closest to an object remains stationary during zooming operation, and which begins with lens units having positive, negative and positive refractive powers.
For example, Japanese Patent Application Laid-Open No. 63-81313 (its U.S. counterpart is U.S. Pat. No. 4,802,747) discloses a zoom lens which includes four lens units having positive, negative, positive and positive refractive powers, and has a variable power ratio of about three. Further, Japanese Patent Application Laid-Open No. 3-296706 discloses as a zoom lens with a higher variable power ratio a zoom lens which includes four lens units having positive, negative, positive and positive refractive powers, and has a variable power ratio of about ten.
On the other hand, as a zoom lens usable in a digital still camera for photo-taking a still picture image using a solid-state pick-up device, it is desired to obtain an optical system which has an extremely short total lens length, and has a wide angle of view in the light of characteristics of the still picture, and optical performance higher than that of a zoom lens used in a video camera for taking a dynamic picture image.
Japanese Patent Publication No. 6-66008 (its U.S. counterpart is U.S. Pat. No. 4,662,723), and so forth disclose a zoom lens which includes two lens units having negative and positive refractive powers, and performs zooming by changing air gaps between lens units, as a lens which contains a wide-angle range, is bright, and achieves high performance though its variable power ratio is relatively low, say about 2.5 to 3.
Japanese Patent Publication No. 7-52256 (its U.S. counterpart is U.S. Pat. No. 4,733,952) discloses a zoom lens which includes three lens units having negative, positive and positive refractive powers, and in which a distance between a second lens unit and a third lens unit is widened during zooming operation from its wide-angle end to its telephoto end.
U.S. Pat. No. 5,434,710 discloses a zoom lens which includes three lens units having negative, positive and positive refractive powers, and in which a distance between a second lens unit and a third lens unit is narrowed during zooming operation from its wide-angle end to its telephoto end.
Japanese Patent Application Laid-Open No. 60-31110 (its U.S. counterpart is U.S. Pat. No. 4,687,302) discloses a zoom lens which includes four lens units having negative, positive, positive and positive refractive powers, and in which a distance between a second lens unit and a third lens unit is narrowed during zooming operation from its wide angle end to its telephoto end, and a fourth lens unit remains stationary during zooming.
Japanese Patent Application Laid-Open No. 10-104520 discloses a zoom lens which includes four lens units having negative, positive, positive and positive refractive powers.
Japanese Patent Application Laid-Open No. 11-84242 (its U.S. counterpart is U.S. Pat. No. 6,191,896) discloses a compact zoom lens which includes four lens units having negative, positive, positive and positive refractive powers, and has a variable power ratio of about three, and in which variation in its exit pupil is relatively small during zooming operation.
U.S. Pat. Nos. 5,872,660 and 6,038,084, Japanese Patent Application Laid-Open Nos. 5-323190, 9-21950, 10-3037 (its U.S. counterpart is U.S. Pat. No. 5,978,154), Japanese Patent Application Laid-Open Nos. 10-206732, 2000-19392 and 2000-89110 (its U.S. counterpart is U.S. Pat. No. 6,308,011), Japanese Patent Application Laid-Open Nos. 2000-275520, 2001-272602 (its U.S. counterpart is U.S. Pat. No. 6,498,687), and so forth disclose a zoom lens which includes lens units having negative, positive and positive refractive powers in the order from the object side, and in which the second lens unit has two aspherical surfaces.
In recent solid-state image pick-up devices, the number of pixels is being increasing, and in conformity therewith a photo-taking lens having higher optical performance is required, as compared with conventional ones.
Generally, in a zoom lens of a negative lead type, where arrangement of refractive powers in respective lens units, and lens constructions and aspherical surfaces of respective lens units are employed, there is a need of appropriately setting a surface on which the aspherical surface is to be formed, and so forth, in order to obtain preferable optical performance over the overall variable power range while reducing the number of lenses in the entire lens system, simplifying the lens construction, and increasing the angle of view.
If the arrangement of refractive powers in respective lens units, lens construction, and selection of places of aspherical surfaces are inappropriate, the effect of the aspherical surface is likely to decrease, and variation in aberrations occurring during zooming operation is liable to increase. Therefore, it is difficult to obtain high optical performance over the overall zoom range.